This invention relates to a method for improving the low temperature impact strength of polyblends comprised of thermoplastic copolyesters and styrenic copolymers. More specifically, it relates to a method wherein an additive is added to a polyblend of a copolyester and a styrene acrylonitrile copolymer resulting in a product with improved low temperature impact strength.
The present invention is related to pending U.S. Pat. No. 4,987,187, filed Dec. 20, 1989 and assigned to the same Assignee, Monsanto Company. That application discloses a blend of copolyester (PETG) and styrene acrylonitrile copolymer (SAN) that produces a product exhibiting good toughness, as demonstrated by impact strength, and good clarity.
Yet for certain applications such as refrigerator crisper pans improved low temperature impact strength is required without an adverse effect on the polyblends clarity.
It has now been discovered, in accordance with the present invention, that the low temperature impact strength of articles molded from the polyblends disclosed in U.S. Pat. No. 4,987,187 is improved by the addition of an effective amount of an additive. These additives are believed to act as dispersants for the SAN. The dispersants are believed to function by breaking up larger domains of SAN and dispersing the SAN phase more uniformly throughout the PETG phase. However, the dispersion activity is only a theory and there is no intent on the part of the inventors to be bound by it. Whether the theory is correct or not, a significant improvement in low temperature impact strength is obtained by the practice of this invention. An improvement is also noted in the mold release characteristics of these polyblends upon addition of an effective amount of dispersant. The clarity of the inventive polyblend is unaffected.
Accordingly, it is a primary object of the present invention to provide a method for improving the low temperature impact strength of polyblends comprised of thermoplastic copolyesters and styrenic copolymers which comprises incorporating therein an effective amount of a dispersant.